Air suspension piston

ABSTRACT

The invention relates to an air suspension piston, with the air suspension piston being tube shaped and being made of aluminium. The air suspension piston is formed by a longitudinal-seam welded tube made from an aluminium alloy, and the tube has been longitudinal-seam welded without any weld metal.

[0001] The invention relates to an air suspension piston according to the generic term of claim 1.

[0002] Air suspension systems are particularly used in the automotive engineering sector, e.g. in the case of automobiles or truck cabins. The characteristic feature of air suspension systems consists of an air cushion element in which air subjected to pressure is being used for cushioning.

[0003] The piston is an important component of an air suspension system. An air bellow, made of rubber, rolls on the outer surface of the piston when the air suspension system is in action and when a cushioning process takes place. The piston positions or supports the air bellow so that there are no folds. Furthermore, the geometry of the piston, namely the selection of various diameters of the piston, has an influence on the spring characteristics of the air suspension system.

[0004] During assembly of an air cushion system, the air suspension piston is fitted into the system components and for this reason the air suspension piston has to be engineered to high precision with minimum tolerances. High precision is also required by the fact that the air suspension piston is continuously exposed to pressure variations as a result of the action of the suspension system, with additional axial forces which act in a dynamic way being superimposed on the pressure variations.

[0005] Usually tubes are used as air suspension pistons; with these tubes being either made of steel or aluminium.

[0006] Insofar as steel tubes are used, they are longitudinal-seam welded and drawn. While such steel tubes meet the requirements of air suspension pistons, in particular concerning their dynamic characteristics, they are however associated with the disadvantage that steel tubes are relatively heavy.

[0007] An effort is therefore made to use aluminium tubes since they are considerably lighter, typically saving approx. 30% weight compared to steel tubes. In automotive engineering there is a general trend to saving weight in the case of dynamically loaded components, because then all the other components whose design is influenced by the vehicle mass, can be of lighter construction. Thus there is an endeavour to use aluminium, in particular if at the same time this leads to a reduction in the unsprung masses, of which the air suspension piston is a part of, and which are so important for driving comfort and.

[0008] Known aluminium air suspension pistons are made from extruded semifinished products. Due to the general precision requirements of air suspension pistons for the reasons explained above, known aluminium tubes have to be drawn. For air suspension pistons, stringent manufacturing tolerances are of prime importance due to sealing against air loss, hence the above-mentioned requirements for precision which relate to both the exterior diameter and the interior diameter as well as the wall thickness of the aluminium tube. Due to the tolerances in the range of a few hundredths of mm applying to the fitted components, conditional on sealing problems as well as acoustic problems in the case of non pressed-on air suspension pistons, exacting requirements apply to the used semifinished products.

[0009] Furthermore the exterior surface must be very smooth, so as too prevent excessive ware of the air bellows which are continuously rolling off this surface.

[0010] Moreover, for reasons of further weight saving, there is an air suspension piston. However, production of respective extruded aluminium tubes of such minimal wall thickness is only possible using a subsequent tube drawing process which is associated with considerable production effort and expense.

[0011] Thus the desired use of aluminium it the production of air suspension pistons is still associated with severe disadvantages and due to the above-mentioned requirements in regard to precision, wall thickness and minimum tolerances, the known extruded and drawn aluminium tubes require a great deal of production effort and expense.

[0012] Apart from extruded aluminium tubes, longitudinal seam welded tubes made of aluminium alloys are know per se. The German published application DE 41 42 325 A1 describes a method for producing chassis members for vehicles, in particular for automobiles, said chassis members being made of aluminium alloys. This published application is based on the fact that in those cases where the conventional extruded aluminium tubes are being processed to become chassis components of motor vehicles, many forming steps are required, including fluid forming or internal high-pressure forming.

[0013] As is described in the above mentioned published application DE 41 42 325 A1, the known extruded aluminium tubes are associated with the disadvantage that fluid forming after previous other forming is not possible because the forming capacity of the material has been exhausted by previous forming. Nor is it possible to carry out intermediate annealing prior to the fluid forming because the material either hardens at low annealing temperatures or becomes very soft at higher annealing temperatures, thus losing its strength. There is no possibility of regaining any of this strength by further heat treatment.

[0014] Against the background of nevertheless finding a starting material for the production of chassis members made of aluminium alloys, with which materials the necessary forming steps can be carried out without any problems, the above mentioned published application DE 41 42 325 A1 proposes a method which uses longitudinal-seam welded tubes from a self-hardened or hardenable aluminium alloy as blanks.

[0015] By contrast, the present invention does not deal with the problems of making possible a multitude of forming steps and of providing for intermediate annealing prior to the fluid forming.

[0016] It is the object of the invention to create an air suspension piston made of aluminium, which in spite of the required precision and close tolerances due to the dynamic characteristics, can be produced economically and cost-effectively, so that the aluminium which per se is an advantageous material, can also be used for the production of air suspension pistons.

[0017] In the air suspension piston stated in the generic term of claim 1, this object is met by the features of the characterising portion of claim 1.

[0018] While up to now, longitudinal seam welded steel tubes were used for producing air suspension pistons, the invention provides for the air suspension pistons to be made from a longitudinal-seam welded tube made of an aluminium alloy.

[0019] Constructing the air suspension piston in the form of a longitudinal-seam welded aluminium tube is of particular advantage. Apart from achieving weight savings compared to steel tubes, the precision and close tolerances which can be achieved with longitudinal-seam welded aluminium tubes are significant. Such air suspension pistons made of aluminium can be produced more economically and cost-effectively than conventional air suspension pistons.

[0020] According to a first development of the invention, the tube is longitudinal-seam welded without any weld metals.

[0021] In a suitable embodiment of the invention, the aluminium alloy consists of a self-hardened alloy.

[0022] The step according to the invention, of constructing them air suspension pistons as longitudinal-seam welded tubes from a self-hardened aluminium alloy, must be considered a surprising measure as the average person skilled in the art knows that higher-strength self-hardened aluminium alloys tend to corrode at temperature above approx. 60 ° C. Yet air suspension pistons are quite commonly exposed to such temperatures. During operation said temperatures result from heat radiation from the engine, from the gearbox, the exhaust manifold and from the brakes.

[0023] The corrosion mentioned is associated with the disadvantage that it can have a significant negative effect on the fatigue strength of the material, which for safety reasons is not permissible in the case of air suspension pistons. Trials show however that the longitudinal-seam welded aluminium tubes according to the invention are not associated with the danger of reduced fatigue strength. Thus, the measures according to the invention surprisingly led to the experts overcoming a prejudice.

[0024] Furthermore, up to now, the average person skilled of the art had considerable misgivings from the point of view of weld seam safety in the case of longitudinal-seam welded aluminium tubes being used where they are exposed to the dynamic loads experienced in air suspension pistons. Weld seam safety is a significant factor in view of the mechanical strength required in the case of components subjected to considerable dynamic loads. For this reason, the state of the art preferably restricted the use of longitudinal seam welded aluminium tubes to components subjected to static loads. While aluminium tubes have been used in the case of components subjected to dynamic loads, these were always extruded aluminium tubes rather than longitudinal-seam welded aluminium tubes.

[0025] In the invention it is advantageous in view of the air suspension pistons, that longitudinal-seam welded tubes welded without weld metal are considerably more precise in relation to wall thickness, and involve considerably smaller tolerances than is the case with extruded tubes. Variations in weight along their length ale significantly smaller in the case of longitudinal-seam welded tubes when compared to extruded tubes made of aluminium. This has an advantageous effect on the dynamic characteristic.

[0026] According to a suitable embodiment of the invention, the weld seam of the tube is raised between −0.1 and +0.1 mm. For production of the air suspension piston according to the invention, high-frequency induction welding (HFI welding) can be used in the known way, with no weld metal being required for the welding process. Nevertheless, a weld seam forms which represents an increase in the wall thickness of the aluminium tube.

[0027] This increase in thickness, referred to as a raised seam, is subsequently removed in the stated tolerance band of—0.1 to 0.1 mm, i.e. in the region of the weld seam, an increased thickness of 0.1 mm is still permissible, on the other hand it is still permissible if during removal of the raised seam too much material is removed, resulting in a reduction in the wall thickness of the tube of 0.1 mm.

[0028] Further suitable embodiments of the invention are provided in the subordinate claims. 

1. An air suspension piston, with the air suspension piston being constructed in the shape of a tube and being made of aluminium, wherein the air suspension piston is a longitudinal-seam welded tube made from an aluminium alloy.
 2. An air suspension piston according to claim 1, wherein the tube has been longitudinal-seam welded without any weld metal.
 3. An air suspension piston according to claim 1 or 2, wherein the aluminium alloy is a self-hardened alloy.
 4. An air suspension piston according to one of claims 1 to 3, wherein the wall thickness of the tube is 1.5 mm.
 5. An air suspension piston according to one of claims 1 to 4, wherein the longitudinal-seam welded tube has been formed to the geometry of the air suspension piston by drawing-in and enlargement operations.
 6. An air suspension piston according to one of claims 1 to 4, wherein the longitudinal-seam welded tube has been formed to the geometry of the air suspension piston by hot forming operations.
 7. An air suspension piston according to one of claims 1 to 4, wherein the longitudinal-seam welded tube has been formed to the geometry of the air suspension piston by not forming operations using internal high-pressure.
 8. An air suspension piston according to one of claims 1 to 6, wherein the weld seam of the tube is raised between −0.1 and +0.1 mm. 